1. Field of the Invention
This invention relates to offshore structures and the risers used to connect such structures to undersea wells, pipelines and the like. More particularly, it relates to catenary risers, including steel catenary risers (SCR's) and catenary risers constructed from other materials like titanium, and the apparatus used to attach a catenary riser to and support a catenary riser from a floating (or fixed) offshore structure.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The top end of a riser, including a catenary riser and including a Steel Catenary Riser (SCR), is typically suspended from a platform (floater, vessel, platform, including a Tension Leg Platform—TLP, spar, buoy, etc) or a platform supported on the seabed (jacket platform, compliant tower etc.). All types of floating structures are referred to herein as floaters. For example, FIG. 1 depicts an SCR suspended from a truss spar floater.
Floaters move about their mean design positions (surge, sway and heave) as well as change their angular orientation with regard to their mean position (pitch, roll and yaw). FIG. 1b illustrates an example of the combined surge and pitch or sway and roll motions of a floater have on the geometry of a catenary riser, which in particular can be an SCR.
The floater motions outlined above are the result of static, dynamic, aerodynamic and hydrodynamic interactions between the floater on its mooring, currents, wind and waves. What is of particular interest here are those interactions that result in large translational and angular offsets of the floater from its mean design positions, like those the example of which is shown in FIG. 1. Those large offsets have typically static and dynamic components. Static offsets are caused by mean currents and winds, while large time variable offsets are caused by dynamic interactions of the floater on its mooring with low frequency wave drift forces and with wind gusts. The low frequency floater motions occur typically with periods of the order of hundreds of seconds. The largest amplitudes of those motions occur where resonance takes place between the fluid dynamic forcing, like wave drift forces or/and wind gust forces, and the vessel mooring. The vessel moored can typically be approximated in each of the 6 degrees of freedom as a damped mass-spring system, whereas the motions for individual degrees of freedom can be fairly independent, or static and/or dynamic couplings can exist between degrees of freedom. These static and dynamic couplings are represented by the existence of non-zero off-diagonal terms in the stiffness and mass matrices of the dynamic system, respectively.
In addition to mean and low frequency motions floaters are also subject to so called first order dynamic motions caused by the floater responses to waves. These motions occur at the wave frequencies, i.e. with periods from a few to a few dozens seconds. For large offshore floaters the motion amplitudes of the said first order motions tend to be smaller than the static and dynamic offsets caused by the mean forces and the low frequency forces.
For simplicity, the floaters are approximated herein as rigid bodies, while the geometries of slender structures such as the risers adjust to the translational and angular offsets of the floater. Riser changes the angles both statically and dynamically due to movements of the hang-off point and due to direct forcing and response of riser to wave and current forces.
In particular, the variation in the relative angle between any orientation on the floater and that of the axis of the riser at the hang-off is of interest herein. The said relative angular floater/riser offsets can result in high bending loads (and stresses), while the translational and combined translational and angular offsets can result in high variations in the effective tensions at the riser hang-offs.
In those cases wherein SCR motions and the said relative angular offsets at the SCR hang-off are not very large, the riser stress variations due to the changes in the said angular offsets and effective hang-off tensions can sometimes be mitigated by adding stress and/or tapered transition joints at the SCR hang-off. These can utilize steel materials, or for larger offsets and stresses titanium alloys can be used. Titanium alloys tend to have higher allowable strains than most typical steel materials used offshore and their Young's Moduli tend to be lower than those of steels. Both the above characteristics of titanium alloys are beneficial for tolerating high angular and translational floater offsets in comparison with the corresponding characteristics of steels.
Materials that are more flexible than steel, like for example Fiber Reinforced Plastics (FRP), can also be used.
In a conventional suspension of the top of an SCR the bending stresses in the SCR are reduced by using a flexjoint, see for example U.S. Pat. No. 5,269,629 (Langner). The use of flexjoints may be combined with the use of tapered or stepped stress joint, etc., which for similar offsets tend to be shorter and have smaller diameters than those required when no flexjoint is used.
A flexjoint comprises flexible (rubbery) elastomeric components that ‘absorb’ the angular deflections. By the said ‘absorbing’, it is meant that most of the bending required occurs by deforming the flexible elastomeric components of the flexjoint thus reducing the amount of bending and the said bending stresses in the metal components of the SCR system. It is noted that elastomeric components of a flexjoint are subjected to pressure and surface action of internal components. Both said pressure action and physical and/or chemical surface action may limit the use of flexjoints in particular due to high pressures, due to thermal, erosive, corrosive, etc. action(s) of internal fluids.
Whenever the said flexjoints and/or said stress joints are used as primary means of reducing bending loads, the effective tension and the bending moment at the hang-off are transmitted to the structure of the floater. These loads do not exert much load on the piping above the flexjoint or stress joint.
Another solution of an SCR hang-off is shown in U.S. Pat. No. 6,739,804 (Haun), which instead of a flexjoint utilizes a universal joint. Unlike with a flexjoint or a stress joint, the said angular offsets are transferred to a pipe spool system above the universal joint. In Haun's design, the spools are provided with piping swivels that allow relative rotations of adjoining segments of the spools, and thus bending and torsional loads and stresses are reduced to relatively small, residual values.
In particular:                Flexjoints are expensive and the maximum SCR pressures are limited; they also allow limited angular deflections.        Flexjoints require the elastomeric material to be exposed to riser contents and pressure.        Piping swivels are subject to leaks, have limited pressure ratings and also may require complicated guiding systems to reduce spool bending on the piping swivels.        
At this time, there is little use of torsional deflection in design for the purpose of stress relieving in offshore pipeline or riser systems. Rigid subsea jumper pipes and pipe expansion spools sometimes incorporate loops, including square loops; ‘L’ or ‘Z’ shapes in order to deal with thermal expansion of pipelines laid on the seabed. The thermal expansion load relief is through increasing bending, shear and in some of these designs also torsional flexibility of the jumper. However, these designs typically see little torsion that is typically incidental to axial and transverse loading of those subsea jumpers that have three-dimensional (3-D) shapes.
There are some patent references to the use of spiral, helicoidal or coil designs and/or some pivoting arrangements in offshore engineering, but those designs are not in widespread use and they do not involve catenary risers. Examples include: U.S. Pat. Nos. 3,189,098, 3,461,916, 3,701,551, 3,718,183, 3,913,668, 4,067,202, 4,137,948, 4,279,544, 4,348,137, 4,456,073, 4,529,334, and 7,104,329.
Catenary riser pipes routinely see limited torque loading that is incidental to any combination of 3-D bending, shear and tension load. Torsional stresses in the catenary risers due to the said torques are usually small in comparison with other loads.
The torsional flexibility of axi-symmetrical members is, however, utilized in mechanical engineering. For example, torsion rods have been used as wheel springs in the suspension of many successful automobiles throughout the twentieth century until now. These do not need to have large dimensions in order to accommodate significant vertical movement of a wheel required that is translated to the torsion of the ‘wheel end’ of the rod.